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Latest news on the Higgs boson

After six hours of presentations dedicated to the search for the Higgs boson at the Moriond conference, here is a summary of the many new results shown today. Both the CMS and ATLAS experiments presented their latest updates, and no matter the angle studied, the new boson is still perfectly compatible with being a Higgs boson. More will be presented next week, once further checks are completed.

The experiments are now trying to establish not only how the new boson decays but also how it is produced. This will eventually help determine if the new boson is really a Higgs boson, either the one prescribed by the Brout-Englert-Higgs mechanism or one associated with supersymmetry, or even not a Higgs boson at all. To answer this question, both teams measured several of the new boson properties, quantities like the signal strength in various production modes, the different decay channels as well as its mass, spin and parity.

Only two decay channels, namely when the boson decays into two photons or four leptons, are used to measure its mass but for all channels, one can measure the signal strength (how many events are found compared to what the Standard Model predicts) and the spin.

An unambiguous signal obtained by the CMS collaboration in the search for a Higgs boson decaying into two Z bosons, each one decaying in turns into two leptons. This is the so-called four-lepton channel. We can see the data (black dots) matching the simulation of a Higgs boson shown by the red line.

The experiments had already checked that the new boson can decay into a pair of other bosons, namely W, Z ou photons, but it had not been established for fermions, the particles of matter like quarks and leptons. This is now a done thing since CMS observes decays into two tau leptons after analyzing the whole data sample. This remains to be proven for b quarks, which might have to wait until more data become available given the high background in this channel. Across the Atlantic though, the Tevatron experiment reported today seeing it at the 3 sigma level, i.e. three times stronger than possible statistical fluctuations.

Other novelty: ATLAS presented the first limit on possible decays of the new boson into invisible particles such as dark matter. This is not expected to happen in the framework of the Standard Model and indeed, with a limit placed at 68% of the time, it is compatible with the model.

The latest signal strength and mass measurements are shown in bold types in the table below along with the older results from last December.

CMS observes a number of events slightly inferior to the expected value in the four-lepton and WW channels while ATLAS reported small excesses for the number of events observed when the new boson decayed either in two photons or four leptons. This is still statistically too weak to draw any conclusion except to notice that all values are still compatible with the Standard Model predictions, all deviations being at most 2.3sigma for ATLAS.

The signal strength for different decay channels as seen by CMS (left plot) and ATLAS (right plot). A value of 1.0 is expected if everything behaves as prescribed by the Standard Model for a Higgs boson.

It will be particularly interesting to see what CMS obtains in the two-photon channel in their next update. If any deviation gets confirmed, it will draw a lot of attention from theorists due the possible huge consequences. A significant deviation with respect to the theoretical predictions would reveal a flaw in the model and help zoom on the right solution.

It is a well-known fact that the current theory, the Standard Model, has its limits. Everyone agrees that there should be a more encompassing theory to describe phenomena like the existence of dark matter, something the Standard Model fails to explain. But what is this new theory is the big question. All attempts so far have failed to find a crack in the Standard Model. Hard to improve on an impressive theory that can make predictions accurate up to the tenth decimal.

New mass measurements were also presented today. No anomalies here either. Last December, with only a third out of the 2012 data sample (21fb-1) analysed, ATLAS had obtained two different mass values for the new boson when measuring it using two different decay channels. Although an impressive series of crosschecks were performed, no experimental mistake was uncovered. The difference was ascribed to a statistical fluctuation. Today, after analyzing the whole data set, the difference is getting smaller, but so are the uncertainty margins. Nevertheless, this is probably a non-issue.

Finally, a few new spin and parity measurements were shown today, such that both experiments observe that the new boson is more compatible with a spin-parity of 0+ as expected for a Higgs boson than with any other spin-parity hypotheses. This is reinforcing the hypothesis that we are indeed dealing with a type of Higgs boson.

CMS checks to see if the new particle is more likely to have a spin-parity of 0+ (in yellow) as expected for a Higgs boson than other hypotheses (all shown in blue). The red arrow shows the value obtained for the new boson. The compatibility with each hypothesis is measured by the amount of the curve lying to the right of the arrow. There is always more yellow remaining than blue, meaning in all cases, the new boson is more likely to have spin 0+ than any other values.

While we are waiting for new results, some of which will be announced next week, you can entertain yourself by watching an animation (or another)recreating how the new boson signal appeared in ATLAS data over time. Meanwhile, as the information is trickling in, the identity of the new boson is slowly being revealed.

Pauline Gagnon

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Andrew

Great overview! One suggestion: you could add the public links to the animations:

Is that animation available outside of CERN? (Link goes to login page.)

http://www.cern.ch CERN

Thanks a lot and sorry about the mistake

http://www.cern.ch CERN

Yes, sorry, I just fixed it now. Thanks for letting me know.

Pauline

Wim de Boer

Hi Pauline,

just a question:

Why did you not update the tau signal strength of CMS (0.9+-0.4)? (Dutt talk)
This is the first time one sees a tau signal with close to 3 sigma significance!
In addition, the bbar signal strength from CMS was given: 1.3+0.7-0.6 (same talk)

Wim

Jon Hart

Has anyone pulled together the data on Spin to give an overall p value for the Boson being Spin=0?

http://www.cern.ch CERN

Hello Wim,

thanks for pointing it out. You are completely right. I meant too but then I finished the blog at 1 am and plain forgot. I will add it now.

As the analyses are refined, I have a strong feeling the photon channel signal strength may be 1.5; 4-lepton channel 1.17; the tau channel 0.83; and the overall signal strength 1.082. I also am of the belief that the spin will be 2 and parity positive. *IF* any of this turns out to be the case, I’d be happy to share how I determined these values.

http://www.cern.ch CERN

Hello William,

thanks for your bets on this. I can see many people like yourself take a keen interest in our work and develop their own models. The equations from the Standard Model are rather complex but of course, everybody is free to play with them. Note though that the scientific community is usually not receptive of models coming from outsiders and based mostly on intuition or symmetry or analogies. These models tend to ignore the whole context and focus on a few simplified aspect, so they are never well received. But your numbers are registered.

I also know many theorists right now are already trying to see what could be modified in the Standard Model picture to create deviations from 1.0, especially for the Higgs to two photon channel even though what is seen in ATLAS right now is too small to be taken seriously. The approach theorists take is to generally add more possibilities in the way the Higgs can decay, i.e. adding new particles like supersymmetric particles. But this is a rather complex exercise. I will report very soon here on one such attempt as soon as the group finishes adding all the recent experimental results to their model.

Cheers, Pauline

http://none Jackie

Amazing stuff, I love reading about it. Just had a eureka moment though. I knew about Higgs Boson etc, but I thought boson was the name of the second man who discovered it! I am clearly an idiot. Hah!

http://lacriolipolisis7.wordpress.com/ acerca de la criolipolisis

You really make it appear really easy together with your presentation but I find this topic to be really something that I feel I’d never understand. It sort of feels too complex and very extensive for me. I’m taking a look ahead on your next put up, I’ll try to get the grasp of it!